Method for the uninterrupted operation of a gas liquefaction system

ABSTRACT

A method for the uninterrupted operation of a gas liquefaction system is provided, wherein the operation is continuously monitored for at least those users of the refrigerant compressor component which represent a two-digit percentage of the total load on the refrigerant compressor component. A total instantaneously available negative load reserve is calculated, and at least one predetermined turbine is switched off when the load reserve reachable via a frequency regulation of the one or more refrigerant compressors is lower than the energy demand of the largest of the refrigerant compressors and either a refrigerant compressor fails or a speed of frequency change for the power supply network for the gas liquefaction system exceeds a present threshold.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International ApplicationNo. PCT/EP2008/058821 filed Jul. 8, 2008, and claims the benefitthereof. The International Application claims the benefits of EuropeanPatent Application No. 07013711.2 EP filed Jul. 12, 2007. All of theapplications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to a method for the uninterrupted operation of agas liquefaction plant, in particular a natural gas liquefaction plant.

BACKGROUND OF INVENTION

The term liquefied natural gas (abbreviated to LNG) is applied tonatural gas which has been liquefied by cooling it. LNG has less than1/600^(th) of the volume of natural gas at atmospheric pressure, and isthus especially suitable for transport and storage purposes; in thisaggregate state it cannot be used as a fuel.

In power plants which are upstream of a plant for the liquefaction oflight carbohydrates, such as for example natural gas, it is conventionalto make use of gas turbines fired by natural gas and if necessary steamturbines in order to provide the required electrical energy from thegenerators coupled to them, which are motor driven.

In conventional natural gas liquefaction plants, the turbo-compressorsfor the refrigerant circuit are driven by directly coupled gas turbines.

Generic disadvantages of these plants are production down times duringthe regular maintenance work which is required on the gas turbines,difficult startup or restart of the compressors with single-shaft gasturbines, together with the direct dependence of the size and the poweroutput of the refrigerant compressor on the type-tested gas turbinesthemselves, the shaft output of which depends in turn on ambientconditions which fluctuate daily or undergo seasonal changes.

For the purpose of avoiding these disadvantages, in newer plants therefrigerant compressor is driven by maintenance-free variable-speedelectric motors. An electric generator driven by a gas or steam turbinesupplies the electrical power for these motors; upstream staticfrequency converters permit a gentle startup and variable-speedoperation. This is then referred to as an eLNG plant (e for electric).

U.S. Pat. No. 7,114,351 B2 describes such a plant for the provision ofthe electrical power for driving the refrigerant compressor of an LNGprocess. By this the electrical power, for the process of liquefyinglight carbohydrates in gaseous form from a source, is provided in afirst step, and in a second step a refrigerant is compressed in arefrigerant compressor which is driven by an electric motor which makesuse of the electrical power generated in the first step.

Electric motors supply their nominal power under various operatingconditions, which permits continuous operation of the refrigerantcompressor even under changing ambient conditions, with a different gas,or input air to the gas turbines at different temperatures. U.S. Pat.No. 7,114,351 B2 also explains that a gas turbine which suddenly goesdown can be replaced by one, or even several, additional gas turbines instandby mode or by one, or even several, turbine sets in standby mode,as applicable. However, the disadvantage of this method is that the LNGproduction process has then already failed, and it takes some hoursuntil the refrigerant compressor concerned has started up again andbecome thermally stable. One must therefore make allowance, inparticular, for interruptions or down times, as applicable.

The applicant's publication “All Electric Driven RefrigerationCompressors in LNG Plants Offer Advantages”, KLEINER et al, GASTECH,Mar. 14, 2005, XP-001544023, therefore proposes a gas liquefaction plantincorporating a power generation module, a transmission module, arefrigerant compression module and a control system, where the powergeneration module has a number of turbine sets and the refrigerantcompression module has at least one refrigerant compressor and a drivemotor, coupled to the refrigerant compressor, for driving electricallythe refrigerant compressor, the transmission module makes available tothe refrigerant compression module the power generated in the powergeneration module, and the control system is connected to the powergeneration module and the refrigerant compression module, where thepower necessary for the rated demand in normal operation can be madeavailable via the control system by partial- or full-load operation ofall the turbine sets, and the number of turbine sets exceeds the minimumnumber which will ensure continuity of operation of the refrigerantcompression module.

The essential thought here is to install a turbine set which isadditional, measured against the total power demand of the eLNG plant,in accordance with the n+1 principle. This turbine set is not a standbyturbine set. In the uninterrupted or normal state of the plant, asapplicable, all the turbine sets necessary for the operation of the eLNGplant, including the n+1^(th) turbine set, work in partial-load mode,i.e. so much spinning reserve is always maintained that it is possibleto compensate for the failure of one turbine set by the controlengineering. In this situation, one or more designated turbine sets canundertake the frequency regulation and in the normal situation all theoperational turbine sets are equally loaded. In the event of theprotective shutdown (tripping) of a turbine or a generator, a controlsystem (dynamic load computer) will decide whether or not measures mustbe initiated for the purpose of stabilizing the stand-alone network.

SUMMARY OF INVENTION

An object of the invention is to specify a method for theinterruption-free operation of a gas liquefaction plant.

In the inventive method for the interruption-free operation of a gasliquefaction plant, incorporating a power generation module, atransmission module, a refrigerant compression module and a controlsystem, where the power generation module has a number of turbine setsand the refrigerant compression module has at least one refrigerantcompressor and, coupled to the refrigerant compressor, a drive motorwith a rated electrical demand, for driving electrically the refrigerantcompressor, the transmission module makes available to the refrigerantcompression module the power generated in the power generation module,and the control system is connected to the power generation module andthe refrigerant compression module, and in normal operation the powernecessary for the rated demand is provided by partial- or full-loadoperation of all the turbine sets, where the number of turbine setsexceeds the minimum number which is necessary to ensure the stability ofoperation of the refrigerant compression module, the operation of atleast those users in the refrigerant compression module which representa two-digit percentage of the total load on the refrigerant compressionmodule is monitored, an overall instantaneously available negative loadreserve is calculated and at least one predetermined turbine is shutdown if the negative load reserve which can be achieved by frequencyregulation of the refrigerant compressor or compressors is smaller thanthe power demand of the largest of the refrigerant compressors andeither a refrigerant compressor fails or a rate of frequency change(df/dt) in the power supply network for the gas liquefaction plantexceeds a prescribed limit.

It is emphasized at this point that, unlike in conventional powernetworks, in the case of stand-alone networks such as for example thepower generation modules of an eLNG plant, the relationship between loadand generator power is such that over 80% of the current load isdistributed across just a few individual loads. This is not the case forconventional networks, where there are very many individual loads with asmall percentage fraction of the total load, and the operation of theconsumers is therefore not observed or monitored.

The best way of all of achieving interruption-free operation of the gasliquefaction plant is by operating the turbine sets in such a way that apositive or negative power reserve which is maintained covers thefailure of the largest turbine machine, whereby the positive powerreserve covers the failure of a generator and the negative power reservethe failure of a motor-compressor train in the refrigerant compressionmodule.

In the event of the failure of a turbine set, the (rotational) speed ofthe compressor drive will preferably be lowered if a previouslydetermined overall positive load reserve is smaller than the power whichwas being supplied by the turbine set before its failure. (According tothe quadratic load characteristic curve of the turbine compressor, thepower drawn from the electric motors reduces as the cube of therotational speed).

If the current energy demand of the refrigerant compression module isnot covered even by the reduction in the compressor drive speed, it isexpedient to switch off at least one predetermined electrical consumerin the gas liquefaction plant (load shedding).

The most far-reaching way of ensuring interruption-free operation of agas liquefaction plant when there are unwanted shutdowns of subsidiaryparts of the plant in the liquefaction process or when predefinedthreshold values are reached by the network frequency and by its rate ofchange, by the voltage and the phase angle in the power supply networkfor the gas liquefaction plant, is by shutting down predeterminedturbines.

The most serious fault to be expected in the operation of an eLNG plantis an unplanned failure of a turbine set in the power generation module,i.e. in the stand-alone power plant—protective shutdowns of compressordrives are subordinate to this in their effects, and in the case ofemergency shutdowns in the process plant it may sometimes be impossibleto maintain operation. However, it is even possible in principle toincorporate a partial emergency shutdown (ESD) of the process plant intothe dynamic load computer's algorithm.

Due to the elimination of maintenance work necessary on the gas turbinesin the power generation module, the duration of interruption-freeoperation for a gas liquefaction plant which this permits in principleis increased, from the previous one to two years up to more than fiveyears. The only remaining obstacle to increasing the expected productivedays from around 340 (the value from historical experience ofdirectly-driven gas liquefaction plants) up to 365 per year is thenunplanned (malfunction) shutdowns.

When variable-speed (converter-fed) electric motors are used and aresupplied with power from a modern gas and steam (combined cycle) powerplant, the thermal efficiency of the plant increases and the emission ofgreenhouse gases is reduced.

By a suitable layout of the drive facilities, the refrigerantcompressors can be started up again, after a process-induced shutdown,within 10 to 30 minutes instead of the 8 to 12 hours for standbyturbines or fixed speed electric motors with start-up converters,without reducing the compressor load and without burning offrefrigerant.

With an appropriate layout of the supplying stand-alone power plant, andintegration of the automation systems involved (e.g. power plant,converter drives, compressors), production from the eLNG plant can alsobe kept interruption-free during a malfunction shutdown of a generatorin the power plant.

Potential dangers to personnel are reduced by the relocation ofmaintenance work out of the explosion-risk process area into the powerplant area.

When variable-speed electric motors with an application-specific layoutare used, it is easier to effect optimization for the process conditionswithin the limitations on the criteria for compressor selection relatingto the rotational speed and power of the gas turbines.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail by way of example withreference to the drawings. These show, schematically and not to-scale:

FIG. 1 the eLNG plant concept

FIG. 2 the control system's load computer algorithm for the positiveload reserve, for realizing a method in accordance with the invention,

FIG. 3 the control system's load computer algorithm for the reduction inthe rotation speed of the compressor modules for realizing a preferredembodiment,

FIG. 4 the control system's load computer algorithm for the shutdown ofpreselected turbines, for realizing a further embodiment,

FIG. 5 turbine utilization in the conventional power generation moduleof a gas liquefaction plant,

FIG. 6 turbine utilization in the power generation module of a gasliquefaction plant with a standby turbine,

FIG. 7 turbine utilization in the power generation module of a gasliquefaction plant with n+1 turbines in partial-load operation, and

FIG. 8 an alternative turbine utilization in the power generation moduleof a gas liquefaction plant with n+1 turbines

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows an integrated solution for a gas liquefaction plant 1 witha stand-alone power plant 23 as the power generation module 2, atransfer module 3 for distributing the power and a refrigerantcompression module 4. A control system 5 is connected to the powergeneration module 2, the transmission module 3 and the refrigerantcompression module 4.

The power generation module 2 incorporates three turbine sets 6, eachwith a turbine 10 and a generator 12, which are connected via a shaft11. However, the power generation module 2 can also incorporate lessthan three or more than three turbine sets 6.

The turbine sets 6 are in each case connected via an electricaltransformer 13 to the power plant busbar 15 of the transmission module3, which makes the electrical power available to the motors in therefrigerant compression module 4 and/or other consumers 26.

In the refrigerant compression module 4, the variable-speed electricmotors 8 of the refrigerant compressor 7 are actuated via convertertransformers 14 and converters 16. Drive motors 8 and refrigerantcompressors 7 are connected via shafts 17, and form motor-compressortrains 9, which finally effect circulation of the refrigerant andcooling of the natural gas 21 in the refrigerant circuit 18.

FIG. 1 shows a schematic representation of the closed refrigerantcircuit 18. Refrigerant compressors 7 transport compressed refrigerantthrough the lines 19 to the liquefaction module 25. Used refrigerant inthe gaseous state is fed back to the refrigerant compressors 7 via lines20.

FIG. 1 shows further an inlet on the liquefaction module 25 for lightgaseous carbohydrates such as, for example, natural gas 21. In theliquefaction module 25 (and other similar stages, not shown here) thenatural gas 21 is transformed by cooling in heat exchangers from thegaseous state into the liquid phase (LNG) 22.

FIG. 2 shows the inventive algorithm of a load computer in the controlsystem 5, for carrying out the method in accordance with the invention,i.e. for controlling the interruption-free operation of a gasliquefaction plant 1. For the purpose of assessing the load conditions,the dynamic load computer receives data 101 constantly from the powerplant management system. The data includes the instantaneous poweroutput from each gas or steam turbine, as applicable, the maximuminstantaneously possible power from each gas or steam turbine, asapplicable, and the minimum instantaneously possible load on each gas orsteam turbine, as applicable, expressed in each case as electricalgenerator power. From the power output and the maximum instantaneouslypossible power, or from the power output and the minimum instantaneouslypossible load, it is possible to determine respectively the positive orthe negative load reserve.

In a first step 102, the dynamic load computer calculates the overallinstantaneously available positive load reserve, taking into accountvarious parameters such as, for example, the instantaneous ambienttemperature, the air humidity, and the calorific value of the combustiongas, which are already taken into consideration in the values 101 fromthe power plant management system.

In a second step 103, the dynamic load computer calculates the positiveload reserve using the power of the largest turbine set 6. If the totalpositive load reserve is sufficient to maintain correct operation of theeLNG plant even if a turbine set 6 is shut down, the dynamic loadcomputer reports to the power station maintenance staff and to the eLNGplant the status “n+1 available” 104. If, in this state, a protectiveshutdown actually does occur in the power plant, the dynamic loadcomputer remains passive, and the power plant management systemreestablishes a balance between the available and demanded loads byreallocating the loads on the remaining generators 12.

If the dynamic load computer determines that the instantaneouslyavailable positive power reserve is not adequate to compensate for anypossible failure of a turbine set 6, it reports the alarm status “n+1not available” 105 to the maintenance office, as a precaution.

This enables the operating staff to mobilize any power reserves whichhave been shut down (e.g. for maintenance work), or to reduce the loadon the network, e.g. by shutting down other consumers 26, and thereby toprevent any interruption in production if a turbine set 6 goes down.Manual load reallocation between the operational turbine sets 6, andchanges in the process steam consumption, are also suitable for thispurpose.

If a precautionary load reduction is not initiated by the operatingstaff of the eLNG plant, e.g. by shutting down unimportant consumers 26or by a temporary reduction in production, the dynamic load computer canintervene, in that it temporarily reduces the speed of all theoperational compressor drives to a value which ensures the stability ofthe compressor, and thereby guarantees the freedom from interruption ofthe production. For this purpose the data 106 received from thecompressor management system, about the load reductions which areinstantaneously possible from reducing the compressor speed withoutendangering the stability of the compressor operation, is continuouslyprocessed and the sum of the possible load reductions for the individualcompression modules is added to the positive load reserve 107. Theoverall load reserve thereby achieved may then possibly cover thefailure of a turbine set 6.

In the alarm status “n+1 not available” it is then possible toreestablish the balance between positive and negative load reserves by alowering of the compressor drive speed. Since this operation can beeffected very quickly, it will only be initiated by the dynamic loadcomputer if a protective shutdown in the power plant actually does takeplace in the alarm state.

The associate algorithm is shown in FIG. 3. As already explained, 107indicates the sum of the positive load reserve of the turbine sets 6 andthe possible load reduction resulting from a reduction in the speed ofthe compressor modules. In the next step 108, the positive load reserveand the possible load reduction are compared with the instantaneouslyavailable power of the largest turbine set 6. Independently of theresult of this comparison, if there is a failure 109 of a turbine 10,the conjunction 110 is true, and the speed of the compressor moduleswill be reduced 111. If the sum of the positive load reserve and thepossible load reduction is less than the power of the largest turbineset 6, or at least the one concerned, there will in addition be loadshedding 112.

Apart from the computational determination of the difference between thepositive and the negative load reserve, it is possible to use anindependent determination of the rate of change of the network frequency(df/dt) for the purpose of recognizing a sudden change in the loadconditions—without regard for its cause. The rate of change of thefrequency is proportional to the step change concerned in the load, andcan thus be used to determine the necessary protective shutdowns.

Since a change in frequency is a direct consequence of the event whichtriggers it, and the determination of the rate of change requires moretime than a protective shutdown via the direct shutdown signals, anyaction based on the calculated frequency change might come too late. Forthis reason, this function can be regarded as a backup to the directshutdown described. Apart from this, it is necessary to ensure thatactions resulting from the computational determination of the lowerfrequency do not cause any spurious tripping.

If the measures described are not sufficient to balance out thedifference between the positive and negative load reserves, the dynamicload computer initiates a chain of preprogrammed load shedding when apredefined lower frequency threshold is reached, in order to prevent afurther fall in the network frequency—and with it a protective shutdownof the entire power plant. The consumers recorded in the load computer,which can if necessary be switched off at times without interruptingproduction, are disconnected from the network as quickly as is required,and to the necessary extent, to maintain the network frequency.

In principle, the algorithm applied to the unplanned shutdown of turbinesets 6 can also be applied to the unplanned shutdown of large consumers,primarily the large compressor drives. The layout of the managementsystem for the power plant and machines is such that it can compensatefor load shedding of this magnitude without the involvement of thedynamic load computer. FIG. 4 shows the principle. If the total of thenegative load reserve which can be achieved by frequency regulation islarger than the largest load shedding to be assumed from shutting downcompressor drives, the dynamic load computer will not intervene.Otherwise, a preselected turbine set 6 will be shut down, and theresulting positive load reserve balances out the remaining gap.

Here, 113 identifies the calculation of the negative load reserve andthe determination of the compressor modules with the largest load. Instep 114, these two values are compared. If the negative load reserve islarger than the larger load from the compressor modules, the computerreports the status “n+1 available” 115. Otherwise it reports “n+1 notavailable” 116.

Using the data from the power plant management system 101 and from thecompressor management system 106, an assignment 117 of turbine sets 6and compression modules is effected. With the help of this assignment,preselected turbines 10 are shut down if the negative load reserve isless 116 than the power demands of the largest compression modules and124 either one compression module goes down 122 or 123 the rate ofchange of the frequency 120 in the power supply network for the gasliquefaction plant 1 exceeds 121 a prescribed limit.

In the case of even larger load shedding 126, e.g. in the case ofpartial emergency shutdowns of the process, it may be necessary to takeseveral turbine sets 6 out of the network 128. If the sequence and thescale 118 of such an emergency shutdown is known, such a procedure canalso in principle be controlled by the load computer, e.g. in that apreselection 119 is made of turbines 10 which are to be shut down, inorder possibly to enable operation of a sub-process to continue. Largeload shedding 126 and the exceeding 121 of a limit for the rate offrequency change 120 are combined together logically in the sense of anon-exclusive disjunction 127.

FIG. 5 shows schematically the turbine utilization in a conventionalpower generation module of a gas liquefaction plant 1, operating asrated. All the turbines 10 of the power generation module run undernominal full load 27. The power generation module operated in this wayprovides no positive load reserve to ensure interruption-free operationof the complete gas liquefaction plant is possible in the event of afailure of a turbine set 6.

FIG. 6 shows schematically the turbine utilization, in the powergeneration module of a gas liquefaction plant operating as rated,described in U.S. Pat. No. 7,114,351 B2. The additional turbine 24, keptready on standby, is started up in the event of a failure of anotherturbine 10 running under full load when the gas liquefaction plant isoperating as rated. Interruptions and down times can be the consequencein the LNG production process in the event of the failure of a turbine10, and it can take a few hours until the refrigerant compressor 7 whichis affected has been started up again and the liquefaction process hasstabilized thermally.

FIG. 7 shows schematically and by way of example the turbine utilizationin the power generation module 2 of a gas liquefaction plant asdescribed in the applicant's publication “All Electric DrivenRefrigeration Compressors in LNG Plants Offer Advantages”, KLEINER etal, GASTECH, Mar. 14, 2005, XP-001544023 when the refrigerantcompression module 4 is operating as rated. All the turbines 10 rununder partial load 28. There is no standby turbine 24. The positive loadreserve is adequate to ensure interruption-free operation of the gasliquefaction plant 1, if a turbine 10 fails, by raising the load on theremaining turbines 10.

FIG. 8 shows schematically and by way of example an alternative turbineutilization in the power generation module 2 of a gas liquefaction plantas described in the applicant's publication “All Electric DrivenRefrigeration Compressors in LNG Plants Offer Advantages”, KLEINER etal, GASTECH, Mar. 14, 2005, XP-001544023 when the refrigerantcompression module 4 is operating as rated. All the turbines 10 rununder partial- or full-load 28,27. Here again there is no standbyturbine 24. However, the utilization of the turbines 10 is notnecessarily the same. Apart from other parameters it is possible, forexample, to take into account the operating life of turbines 10 indetermining their utilization on a machine-specific basis.

1.-4. (canceled)
 5. A method for interruption-free operation of a gasliquefaction plant comprising a power generation module including aplurality of turbine sets; a transmission module providing powergenerated in the power generation module to the refrigerant compressionmodule; a refrigerant compression module including a refrigerantcompressor and a drive motor with a rated electrical demand coupled tothe refrigerant compressor as an electrical drive for the refrigerantcompressor; and a control system, the control system being connected tothe power generation module and to the refrigerant compression module,and in normal operation the power required for the rated demand isprovided by partial- or full-load operation of all the turbine sets,wherein the plurality of turbine sets exceeds a minimum plurality ofturbine sets necessary to ensure continuity of operation of therefrigerant compression module, the method comprising: monitoringcontinuously the operation of at least those consumers in therefrigerant compression module representing a two digit percentagefraction of the total load from the refrigerant compression module;calculating a total instantaneously available negative load reserve; andshutting down at least one predetermined turbine when the negative loadreserve achievable by frequency regulation of the refrigerant compressoris less than the power demand from the largest of the refrigerantcompressors and the refrigerant compressor goes down.
 6. The method asclaimed in claim 5, wherein an instantaneously available positive loadreserve is calculated and a compressor drive speed is lowered in theevent of the failure of a turbine set when the positive load reserve isless than the power provided by the turbine set before the failure. 7.The method as claimed in claim 6, wherein at least one predeterminedelectrical consumer in the gas liquefaction plant is shut down when,after the failure of a turbine set, even a reduced compressor speed doesnot enable the actual power from the turbine sets to cover the currentpower demand for the refrigerant compression module.
 8. The method asclaimed in claim 5, wherein predetermined loads are shed when predefinedlower threshold values for the network frequency are reached in thepower supply network for the gas liquefaction plant.
 9. The method asclaimed in claim 6, wherein predetermined loads are shed when predefinedlower threshold values for the network frequency are reached in thepower supply network for the gas liquefaction plant.
 10. The method asclaimed in claim 7, wherein predetermined loads are shed when predefinedlower threshold values for the network frequency are reached in thepower supply network for the gas liquefaction plant.
 11. A method forinterruption-free operation of a gas liquefaction plant comprising apower generation module including a plurality of turbine sets; atransmission module providing power generated in the power generationmodule to the refrigerant compression module; a refrigerant compressionmodule including a refrigerant compressor and a drive motor with a ratedelectrical demand coupled to the refrigerant compressor as an electricaldrive for the refrigerant compressor; and a control system, the controlsystem being connected to the power generation module and to therefrigerant compression module, and in normal operation the powerrequired for the rated demand is provided by partial- or full-loadoperation of all the turbine sets, wherein the plurality of turbine setsexceeds a minimum plurality of turbine sets necessary to ensurecontinuity of operation of the refrigerant compression module, themethod comprising: monitoring continuously the operation of at leastthose consumers in the refrigerant compression module representing a twodigit percentage fraction of the total load from the refrigerantcompression module; calculating a total instantaneously availablenegative load reserve; and shutting down at least one predeterminedturbine when the negative load reserve achievable by frequencyregulation of the refrigerant compressor is less than the power demandfrom the largest of the refrigerant compressors and a rate of change inthe frequency in the power supply network for the gas liquefaction plantexceeds a prescribed limit.
 12. The method as claimed in claim 11,wherein an instantaneously available positive load reserve is calculatedand a compressor drive speed is lowered in the event of the failure of aturbine set when the positive load reserve is less than the powerprovided by the turbine set before the failure.
 13. The method asclaimed in claim 12, wherein at least one predetermined electricalconsumer in the gas liquefaction plant is shut down when, after thefailure of a turbine set, even a reduced compressor speed does notenable the actual power from the turbine sets to cover the current powerdemand for the refrigerant compression module.
 14. The method as claimedin claim 11, wherein predetermined loads are shed when predefined lowerthreshold values for the network frequency are reached in the powersupply network for the gas liquefaction plant.
 15. The method as claimedin claim 12, wherein predetermined loads are shed when predefined lowerthreshold values for the network frequency are reached in the powersupply network for the gas liquefaction plant.
 16. The method as claimedin claim 13, wherein predetermined loads are shed when predefined lowerthreshold values for the network frequency are reached in the powersupply network for the gas liquefaction plant.
 17. A gas liquefactionplant, comprising: a power generation module including a plurality ofturbine sets; a transmission module providing power generated in thepower generation module to the refrigerant compression module; arefrigerant compression module including a refrigerant compressor and adrive motor with a rated electrical demand coupled to the refrigerantcompressor as an electrical drive for the refrigerant compressor; and acontrol system, the control system being connected to the powergeneration module and to the refrigerant compression module, and innormal operation the power required for the rated demand is provided bypartial- or full-load operation of all the turbine sets, wherein theplurality of turbine sets exceeds a minimum plurality of turbine setsnecessary to ensure continuity of operation of the refrigerantcompression module.